Isocyanate-terminated, polyurethane prepolymers (sometimes referred to hereinafter as polyurethane prepolymers) are desirable in a variety of applications. For example, they can be used in reactive hot melt urethane adhesive, coating and/or sealant systems. These hot melt urethane systems are solid at room temperature, melt to a viscous liquid when heated to moderate temperatures (82.degree.-121.degree. C.), and are applied in a molten state. They then cool to a solid state to provide initial bond strength (that is, "green strength"), and, additionally, cure on contact with ambient moisture to provide their final bond strength.
A number of reactive urethane hot melt compositions are known. For example, U.S. Pat. No. 3,931,077 discloses a reactive hot melt composition comprising (a) a urethane prepolymer prepared from a diol selected from ethylene glycol, propylene glycol, 1,4-butanediol, 1,4-butinediol (sic), polymers thereof, and polytetramethylene glycolether (sic), (b) a thermoplastic resin of an ethylene-vinyl copolymer, an ethylene-acrylic acid copolymer, an ethylene-acrylate copolymer, an atactic polypropylene or a poly(ethyleneterephthalate) linear polymer, and (c) a tackifier.
U.S. Pat. No. 4,352,858 discloses a modified polyurethane adhesive composition comprising an isocyanate-terminated prepolymer and a dicarbamate ester which is compatible with the prepolymer and which contains two free isocyanate groups.
U.K. Patent No. 2,137,638 discloses a urethane hot melt adhesive composition which comprises a moisture-curable polyurethane formed from a diisocyanate, a substantially linear hydroxy polyester, and a monofunctional reactant present in an amount sufficient to react with up to 40 mole percent of the isocyanate groups of the product of the prepolymer reaction.
U.S. Pat. No. 4,585,819 discloses a polyurethane hot melt composition which comprises an isocyanate prepolymer, a thermoplastic polyurethane or polyester and/or a low molecular weight synthetic resin selected from the group consisting of ketone resins, hydrogenation products of acetophenone condensation resins, and mixtures thereof.
European Patent Application Publication No. 0 340 906 discloses a hot melt polyurethane adhesive composition which comprises a mixture of at least two amorphous polyurethane prepolymers characterized in that each prepolymer provides a different glass transition point to the composition.
U.S. patent application Ser. No. 07/201,169 discloses polyurethane hot melt adhesives, coatings and sealant systems which employ mixtures of isocyanate-terminated polyurethane prepolymers prepared from hydroxy-functional polyesters. These mixtures do not, however, encompass the mixtures of the present invention. Moreover, the mixtures of the present invention provide better adhesion to a variety of substrates, including certain plastic substrates, than do those of the pending application.
The compositions of the invention are an improvement over these prior compositions in that they achieve adhesion to substrates even though they are 100% reactive systems. Thus, while various adjuvants can be added to the compositions of the invention, their addition is unnecessary to the attainment of adhesion.